Imaging apparatuses that irradiate a closely situated object with illumination light and photographing the object have been used for biometric authentication and monitoring. For example, as shown in FIG. 15, in a photographing environment where the biometric authentication is performed, there exists ambient (external) light (or environmental light) other than light emitted from its own illumination provided in the imaging apparatus. Accordingly, an angle and intensity toward an object cannot be controlled, thereby increasing a difference between an image to be authenticated photographed in such a photographing environment, and a correlation image photographed in a photographing environment without an influence of ambient light. As a result, the accuracy of authentication may be affected. Therefore, the photographing environment for performing biometric authentication is limited to the environment without ambient light or to that with weak ambient light.
In response, for example, as shown in FIGS. 16 and 17, a technology is disclosed for acquiring an image from which an influence of ambient light is eliminated, by calculating a difference between an image photographed by turning on its own illumination, and an image photographed while leaving its own illumination turned off. In the technology disclosed in, for example, Japanese Laid-open Patent Publication No. H17-19823.
A technology is also disclosed in which a view finder of a video camera that becomes difficult to see due to the influence of ambient light is made viewable, by collecting ambient light in the apparatus and actively using the ambient light as a backlight. See, for example, Japanese Laid-open Patent Publication No. H7-95451.
However, with the conventional technologies, the quality of a photographed image maybe deteriorated, or the collected ambient light may sometimes be unsuitable for illumination, etc.
In other words, with the conventional technology that acquires an image from which the influence of ambient light is eliminated, by calculating a difference between an image photographed by turning on its own illumination and an image photographed while leaving its own illumination turned off, for example, as shown in FIG. 18, the contribution of its own illumination relatively decreases with the increase of ambient light. Accordingly, a signal-to-noise (S/N) ratio of an image deteriorates.
With the conventional technology in which a view finder of a video camera that becomes difficult to see due to the influence of ambient light is made viewable, by collecting ambient light in the apparatus and actively using the ambient light as a backlight, the collected light is used. However, with an imaging apparatus used for biometric authentication, characteristics of light such as wavelength are sometimes unsuitable for its own illumination.